Alkaline cells are well known in the art and generally employ a zinc anode, manganese dioxide as the cathode with an aqueous solution of potassium hydroxide for the electrolyte. These cells are readily available commercially for industrial and home applications. An improved type of alkaline cell has been disclosed by Cegasa International, a Spanish company. This cell, referred to as an air-assisted cell, employs zinc as the anode and manganese dioxide as the cathode with an aqueous solution of potassium hydroxide as the electrolyte. This cell is designed so that the positive electrode containing the manganese dioxide (MnO.sub.2) is supported about is periphery and along its full length in the cell by a perforated ribbed air distribution grid. The bottom or negative end of the cell has an insulating support which allows air to enter the cell and pass up along the outside of the supported positive electrode. When the cell is initially put into a circuit, the electrochemical reaction depends primarily upon the presence of the manganese dioxide cathode. As the reaction progresses, and the manganese dioxide cathode is electrochemically reduced, air within the cell reoxidizes and recharges the manganese dioxide. Thus an air-assisted cell is designed to use oxygen in the air to "recharge" manganese dioxide in the cathode. This "recharging" of the manganese dioxide means that the fixed quantity of manganese dioxide in the cathode can be discharged and then recharged numerous times.
Standard alkaline batteries and air-assisted alkaline batteries have traditionally been made with mercury in the anode. Mercury helped to prevent gassing by raising the hydrogen overvoltage. Due to concerns about the environment, battery manufacturers are designing alkaline batteries with little or no mercury. As the percentage of mercury in the cells has been reduced, problems with leakage and decreased shelf life have become apparent. The reduction or elimination of mercury has led to service and shelf storage problems because gassing in and the expansion of the anode in the anode compartment have forced electrolyte to flow from the anode compartment into the cathode compartment. As electrolyte moves from the anode into the cathode, ionic conductivity in the anode decreases and the cell cannot discharge efficiently. If the anode compartment continues to gas and expand, the electrolyte from the anode could be driven into the cathode and may essentially be driven through the seal and out of the cell.
U.S. Pat. No. 4,857,424 discloses an alkaline zinc-manganese dioxide electrochemical cell containing a small amount of an organosiliconate additive as a means for reducing the amount of mercury used in the zinc amalgam. The organosiliconates added to the anode are film forming organic wetting agents and can be used in mercury containing anoidic zinc gels containing other gas suppressors.
U.S. Pat. No. 4,195,120 discloses that hydrogen evolution in cells having zinc anodes is reduced or eliminated by incorporating in the cell a surfactant which is a complex phosphate ester of a surfactant of the ethylene oxide-adduct type. This surfactant is added in such a manner that, directly or upon wetting of the anode by the electrolyte, there is an adsorption of surfactant on the surface of the zinc anode material, whereby hydrogen evolution is inhibited. The surfactant is desirably present in the cell in an amount of from 0.001% to 5% by weight of the zinc component of the cell. Useful surfactants disclosed in this patent are organic phosphate esters. Such surfactants will generally be a monoester or a diester having the following formula: ##STR1## where x+y=3;
M=hydrogen, ammonia, amino, or an alkali or alkaline earth metal; and PA1 R=phenyl or alkyl or alkylaryl of 6-28 carbon atoms. Specific useful phosphate ester surfactants include materials which can be identified by their commercial designation as GAFAC RE610, GAFAC RA600 and KLEARFAC AA-040. PA1 R is an alkyl radical which may optionally be branched, an aryl radical, or an alklaryl radical; and PA1 n lies in the range 2 to 100. PA1 N is between 4 and 20, preferably between 6 and 8; PA1 p is between 1 and 10 and is preferably close to 2; and PA1 q is between 3 and 40, preferably between 10 and 12. A single compound may be used corresponding to the above formula or else a mixture of two or more compounds corresponding to this formula may be used. It is also possible to use a non saturated perfluorated organic compound of formula: EQU C.sub.n F.sub.2n-1 --(CH.sub.2).sub.p --(CH.sub.2 CH.sub.2 O).sub.q --OH PA1 a+b=3; PA1 n is 3; PA1 m is 13;
U.S. Pat. No. 4,847,669 discloses the use of an ethylene oxide polymer in the cell in an amount sufficient to reduce the rate of reaction between the zinc and the alkaline electrolyte, thereby permitting a significant reduction in the proportion of mercury needed to achieve satisfactory shelf life and thereby provides a means for eliminating some of the ecologically objectionable mercury from the cells and replacing it with ecologically innocuous materials.
U.S. Pat. No. 4,781,999 discloses a method of stabilizing a primary electrochemical cell which includes at least one reactive anode made of a metal selected from the group constituted by zinc, aluminum, and magnesium such that the method consists in adding a stabilizer to said electrode, the stabilizer concentration lying in the range 0,001% to 1% by weight of electrode metal, and the stabilizer comprising at least one alkyl and polyethoxylalcohol sulfide satisfying the following formula: EQU R--S--(CH.sub.2).sub.2 --(O--CH.sub.2 --CH.sub.2 --).sub.n --OH(1)
in which:
U.S. Pat. No. 4,606,984 discloses a process for stabilizing a primary electrochemical generator comprising at least one reactive anode made from a metal chosen from the group comprising zinc, aluminum and magnesium, characterized in that to said electrode is added a percentage of 0.01% to 1% by weight with respect to the metal, of at least one perfluorated organic compound of the ethoxylated fluoroalcohol type. A preferred perfluorated compound disclosed has the formula: EQU C.sub.n F.sub.2n+1 --(CH.sub.2).sub.p --(CH.sub.2 CH.sub.2 O).sub.q --OH
in which
in which n, p and q have the same meaning as the above formula.
Even with the reduction of mercury in alkaline cells, it has been found that when using a brass anode current collector in the cell, undesirable gas may be formed due to the presence of the brass anode current collection. Many different types of brass materials have been used in alkaline cells with some being better than others. However, the use of a brass anode current collector in alkaline cells lead to the formation of undesirable gas when the cell contains little or no mercury.
It is an object of the present invention to provide an additive for alkaline cells employing little or no mercury and a brass anode current collector.
It is another object of the present invention to provide an additive to an alkaline cell employing little or no mercury and a brass anode current collector in which said additive is a specific phosphate ester.
These objects, together with other and further objects of the invention which appear from the following description, are accomplished by the invention of the alkaline zinc-manganese dioxide electrochemical cells described herein.